1. Field of the Invention
This present invention relates to a semiconductor device provided comprising a semiconductor chip, gold (Au) bumps and copper (Cu) leads, a method of fabricating such a semiconductor device, and Cu leads. More specifically, the present invention relates to a semiconductor device that enables firm bonding of Au bumps to Cu leads, a method of fabricating such a semiconductor device, and Cu leads.
2. Description of the Related Art
The semiconductor chip of a conventional semiconductor device to be connected to leads by a tape-automated bonding method (TAB method) generally is provided with bumps, i.e., metal electrodes, on electrode pads. As shown in FIG. 20, a conventional semiconductor device 1 has a semiconductor chip la provided with electrode pads 2. In FIG. 20, only one of the electrode pads 2 is shown. The surface of the semiconductor chip la on which the electrode pads 2 are formed is coated entirely with a passivation film 3 of Si.sub.3 N.sub.4 or SiO.sub.2, and the surfaces of the electrode pads 2 are exposed partially through openings 4 formed in the passivation film 3, respectively.
A barrier metal layer 5 is formed so as to cover the exposed portion of each electrode pad 2, and an area of the passivation film 3 around the edge of the opening 4.
Generally, the barrier metal layer 5 consists of a plurality of metal thin films. The barrier metal layer 5 shown in FIG. 20 by way of example is formed by sequentially forming a first barrier metal layer 5a of titanium (Ti), a second barrier metal layer 5b of nickel (Ni) and a third barrier metal layer 5c of palladium (Pd). A bump 6 is formed on the third barrier metal layer 5c. Generally, the bumps 6 of semiconductor devices to be mounted on a TAB tape by a TAB method are formed of Au.
As shown in FIG. 20, an inner lead 7, i.e., a portion of a Cu lead of a TAB tape, is bonded to the bump 6 by a bonding process, i.e., an inner lead bonding process (ILB process). The Cu leads are coated with a 0.4 to 0.6 .mu.m thick plated tin (Sn) layer.
The Cu lead coated with the plated Sn layer has an inner lead 7 and an outer lead. The thickness of a portion of the plated Sn layer on the side of the outer lead of the Cu lead must be sufficiently large for reliable outer lead bonding; otherwise Cu appears on the surface of the outer lead, which will lead to entailing problems in oxidation of Cu and then less wettability during bonding. Therefore, the thickness of the plated Sn layer coating the outer lead of the Cu lead must be in the range of 0.4 to 0.6 .mu.m, and hence, generally, the thickness of the plated Sn layer on the inner leads is in the range of 0.4 to 0.6 .mu.m as mentioned above.
When bonding together the inner lead 7 and the Au bump 6 by inner lead bonding (ILB), the the inner lead 7 is brought into contact with the Au bump 6, and then heat and pressure are applied to the interface between the inner lead 7 and the Au bump 6. When the inner lead 7 and the Au bump 6 are thus bonded together, fillets 8 are formed on the side surfaces of the inner lead 7, and an alloy layer 9 is formed in the interface between the inner lead 7 and the Au bump 6. The fillet 8 and the alloy layer 9 secure the bonding strength between the inner lead 7 and the Au bump 6.
Incidentally, the fillet 8 and the alloy layer 9 formed when the Au bump 6 and the inner lead 7 coated with the plated Sn layer are bonded together are formed of An Au--Sn alloy; the fillet 8 is formed mainly of a eutectic Au--Sn alloy (70.7 at. % Au and 29.3 at. % Sn (M. Hansen, "Contribution of Binary Alloys", Genium Publishing Corp., New York (1985)), and the alloy layer 9 is formed of a eutectic Au-Sn alloy or a zeta-phase Au-Sn alloy (84 to 88 at. % Au and 12 to 16 at. % Sn) (Bike Zakel, et al., 42nd ECTC Proceeding, pp.360-371, (1992)). The alloy layer 9 is formed of such an Au--Sn alloy because the plated Sn layer coating the inner lead 7 has a comparatively large thickness of 0.4 to 0.6 .mu.m and and hence it is difficult for Cu forming the inner lead 7 to appear on the surface of the plated Sn layer.
The semiconductor device is subjected to accelerated durability tests, i.e., reliability tests, to evaluate the reliability of the bonding parts. A high-temperature stability test is a representative accelerated durability tests. The high-temperature stability test uses a property of metals that the diffusion rate of metals at a high temperature is higher than that at an ordinary temperature. During the high-temperature stability test, Cu forming the inner lead 7 diffuses into the fillets 8 formed on the side surfaces of the inner lead, forming voids (defects) in the Cu inner lead 7 by the Kirkendall effect. According to studies made by Zakel, et al., Cu forming the inner lead 7 diffuses into the alloy layer 9, causing the alloy layer 9 originally being of an Au--Sn alloy of a binary alloy system to change into an Au--Cu--Sn alloy of a ternary alloy system.
When the alloy layer 9 changes into such an alloy of a ternary alloy system, voids are formed in the surface of the inner lead 7 on the side of the semiconductor chip la due to the Kirkendall effect. If the voids grows, it is possible that the inner lead 7 is disconnected from the Au bump 6.
Zakel, et al. made a detailed report on the condition of bonding parts formed by ILB after the bonding parts had been kept in a high-temperature environment in Elke Zakel, et al., 42nd ECTC Proceeding, pp. 360-371 (1992). In experiments conducted by Zakel, et al., the plated Sn layer coating the inner lead 7 of a TAB tape was formed in a thickness of 0.7 .mu.m by an electroplating process, the heating temperature for ILB was 400 to 500.degree. C. and the ILB pressure applied to the bumps was 10 or 40 cN (10 or 40 gf). When the ILB pressure is comparatively low, a eutectic Au--Sn alloy is formed in the alloy layer 9. During the high-temperature stability test, Cu of the inner leads 7 diffuses into eutectic Au--Sn portions of the alloy layer 9, and voids are formed by the Kirkendall effect when the Au-Sn alloy changes into an Au--Cu--Sn alloy.
It is said that a zeta phase is formed mainly in the alloy layer 9 and Cu of the inner lead 7 does not diffuse into the alloy layer 9 during the high-temperature stability test when the ILB pressure is comparatively high, which is inferred to be due the extrusion of the eutectic Au--Sn alloy formed during ILB from the bonding part by the comparatively high ILB pressure and the resultant formation of a zeta phase in which the Sn content is comparatively small. Zakel, et al. concluded that the zeta phase thus formed serves as a barrier against the diffusion of Cu to suppress the formation of voids and, consequently, the reliability of the bonding part is enhanced. However, it is unavoidable that an Au--Sn alloy having a high Sn content remains locally in the bonding part and voids are possible to grow from portions of the bonding part having a high Sn content.